This invention relates generally to power control circuits for induction motors and more particularly to means useful in such circuits for obtaining signals accurately representative of the motor's rotor-stator air gap flux.
In the control of traction vehicle speed, it is desirable to regulate the torque output of the system and in particular to maintain a predetermined torque capability at the output shaft of the motor. This can readily be accomplished if the rotating magnetic field strength, or the motor air gap flux level is determined. In the case of an a-c motor, as the motor speed requirements change, it is necessary to change the frequency of the power supply, and with that the amplitude of the motor excitation must be accordingly varied in order to maintain the desired flux level. Thus, there are a number of different types of voltage control systems for controlling the volts-per-hertz ratio in accordance with the particular a-c motor requirements. However, they all have certain disadvantages, among which is the necessity for a significantly large flux margin which reduces the torque efficiency of the motor. The flux margin is further increased by the control errors which are introduced by motor parameter changes (i.e. temperature, etc.). For example, in order to maintain constant flux, the motor voltage is controlled with reference to model characteristics of the motor. Since all the characteristics cannot be precisely included in the model, a margin of error will be introduced; and when the motor parameters change from the model characteristics, the margin of error will be increased.
Also when controlling voltage without reference to a model motor, the theoretical flux level for a single voltage is different for motoring and braking. During motoring the net flux is less than would ideally result from the applied voltage since there is a loss of power due to motor resistance and reactance. However, during braking where the motor acts as a generator, the opposite is true, i.e. the voltage builds up and the resulting flux is greater than would ideally be the case. Also, when braking at low speeds the IR drop causes stability problems in the system. This occurs because both the source and the motor are producing power to supply the internal motor losses.
Another requirement for voltage control systems using motor model characteristics is the requirement for an accurate knowledge of motor slip frequency. This can only be obtained by the use of a digital tachometer and digital frequency control when a low slip motor (for high efficiency) is used.
It is therefore an object of this invention to provide an a-c motor propulsion system capable of producing substantially controlled flux at variable motor speed.
Another object of this invention is the provision in an a-c motor for regulating the air gap flux at a substantially controlled value over the full load range.
Yet another object of this invention is the provision for a flux regulation system whose air gap flux level is not affected by motor parameter changes.
Still another object of this invention is the provision for an a-c motor flux regulation system which is equally effective during motoring and braking operations.
A further object of this invention is the provision for a flux regulation system that is not affected by IR drop at low speeds of operation.
Yet another object of this invention is the provision for a flux regulation system which does not require accurate knowledge of motor slip frequency.
Still another object of this invention is the provision for a flux regulation system which is economical to manufacture, simple in principle and extremely effective in use.
These objects and other features and advantages become more readily apparent upon reference to the following description when taken in conjunction with the appended drawings.